Over the years, the construction of blades for wind turbines has developed towards a shape, where the blade (when mounted on the wind turbine) consists of a root area closest to a hub of the wind turbine, a profiled area or airfoil area furthest away from the hub and a transition area between the root area and the airfoil/profiled area. The airfoil area has an ideal or almost ideal blade shape, whereas the root area has a substantially circular cross-section, which reduces the loads from gusts of air and makes it easier and safer to mount the blade to the hub. The root area diameter is typically constant along the entire root area. As is suggested by the name, the transition area has a shape gradually changing from the circular shape of the root area to the airfoil profile of the airfoil area. Typically, the width of the transition area increases substantially linearly with increasing distance from the hub.
As mentioned, the profiled area or airfoil area has an ideal or almost ideal blade shape, for instance shaped as a typical aeroplane wing. Typically, the cross-section of the blade in the airfoil area is shaped as a typical airfoil profile with a suction side and a pressure side and a chord extending, as seen transversely to the longitudinal direction of the blade, between a leading edge and a trailing edge, where the leading edge is rounded with a given nose radius and the trailing edge has an essentially pointed shape. However, it is also possible to reduce the width of the blade by truncating the blade in the area of the trailing edge, the cross-section thus forming a truncated profile of an imaginary airfoil with an essentially pointed trailing edge. In this situation, the air flows departing at the truncated part of the blade from the pressure side and the suction side, respectively, can meet each other at the trailing edge of the imaginary airfoil profile. Thus, the air flows “see” the truncated profile as the imaginary profile. Such a truncated profile has the advantage that the transverse surface area of the blade profile is reduced, thus reducing loads from wind gusts but at the same time maintaining an “aerodynamic width”, which corresponds to the chord length of the imaginary profile. The total lift generated by the truncated profile is substantially the same as a profile without the truncation, since the part of the blade generating the highest lift has been maintained. Additionally, the strength of the trailing edge of the blade profile is increased due to the profile being thicker at the trailing edge. Such a blade is for instance disclosed in DE19614420.
Wind turbine customers have different demands to the size and functionality of the wind turbine depending on the intended use and the intended place of operation. Therefore, the demands to the size, such as length and solidity, as well as functionality, such as lift and drag coefficients, of wind turbine blades also varies greatly. Consequently, manufacturers of wind turbine blades need to have a large number of different moulds for producing the wind turbine blades, which are typically manufactured as a shell member of fibre-reinforced polymer.
US 2006/280614 A1 discloses a rotor blade fitted with stall fences in form of planar elements protruding from the suction side of the rotor blades in zones of a transversal flow.
DE 199 64 114 A1 discloses an airfoil profile, which is fitted with a divergent trailing edge in form of Gurney flap, which creates a periodic flow disturbance.
EP-A-1 314 885 discloses a wind turbine blade provided a flexible serrated trailing edge. The document discloses an embodiment with a Gurney flap.
DE 198 54 741 C1 discloses a trailing edge wedge for an aircraft wing, which is attached at the downwardly facing wing surface at the trailing edge. The edge wedge compensates for asymmetric aircraft characteristics due to unavoidable manufacturing tolerances.